Typical optical taps that are commonly seen in the market are those that use optical fibers to achieve optical tapping. One method of obtaining an optical tap with fibers is done by fusing two fibers together, whose tips are cleaved and polished. By varying the cleaved angle during fusion splicing, or by the use of materials (such as TiO2 or Al) with different indices of refraction, variable tap ratios can be obtained [Noureddin M. Ibrahim and Ismail H. A. Fattah, “Narrow-beam aluminium-mirrored fiber optical-taps with controllable tapped power,” IEEE Journal of Selected Topics In Quant Electronics., Vol. 2, No. 2, pp. 221-225, June 1996]. Another method makes use of the fused biconical fiber technology, whereby control of the tap ratio is through the fiber structure and dimensions during fusion splicing [D. R. Moore, Z. X. Jiang and V. J. Tekippe, “Optimization of tap couplers made by the FBT process,” Technical Paper, Gould Fiber Optics], [Tzong-Lin Wu, Hug-chun Chang, “Vectorial analysis of fiber-core effects in weakly fused couplers”, IEEE Photonics Technology Letters, Vol. 9, No. 2, February 1997.].
A further method utilises a bend fiber/prism/photodiode assembly [John P. Palmaer, “Fiber optic tap and method of fabrication,” U.S. Pat. No. 0,439,875]. In this method, an optical fiber is bent to increase the tendency for light to escape from the bent region. By lapping and polishing the bent region perpendicular to the radius of the bend, it is possible to enhance the tendency for light to escape from the bend. A prism is then used to direct the light to the associated photodiode detector. The tap ratio is controlled by the depth of lapping into the fiber.
However, the fused fiber optical tap has the disadvantages of being difficult to manufacture and it is sensitive to manufacturing tolerances. Precise machinery is needed to control the fabrication parameters, such as thickness of the thin film, radius of the fiber core and lapping depth. Also, repeatability of the manufacturing process is an issue, for example, properties of thin films, like TiO2, used in the manufacture of optical taps have wide variation even among those produced by the same deposition process. Moreover, fiber based taps are basically discrete components and thus are bulky. The coupling region can extend to several millimeters and the packaged tap/coupler can be several centimeters in length. According to the present invention, a new type of optical tap based on semiconductor technology is proposed.